The test, developed at University of Southern California (USC), "appears to be a very powerful test and better than anything else we know of for predicting recurrence," says Richard Cote, professor of pathology and urology at the Keck School of Medicine of USC.

Cote and his colleagues looked at 3,914 lymph nodes from 180 patients who were staged as having lymph nodes negative for cancer based on standardised histologic evaluation. The lymph nodes were then evaluated for occult metastases using new specific immunohistochemistry tests that can detect cancer on a cell-by-cell level.

Their new analysis checks for cells that react with antibodies to cytokeratins and PSA. The team's testing found occult tumour cells in the lymph nodes of 24 of the patients whose lymph nodes had been previously been diagnosed as cancer-free. The test used to detect the occult tumour cells is more sensitive than any clinical, pathologic or radiographic techniques, Cote says.

The group compared cancer recurrence and survival in those patients with the hidden tumour cells versus those without the cells. The presence of occult tumour cells was associated with increased prostate cancer recurrence and decreased survival. In fact, "the outcome for patients with occult tumour cells was similar to those who were identified as having positive lymph nodes at the time of the surgery," Cote says.

"We have shown that occult tumour spread in lymph nodes is a significant predictor of disease recurrence," he says. "Once surgery is performed, the primary form of treatment is adjuvant systemic therapy. In patients with no evidence of metastasis, success of such therapy is assumed to be due to killing of occult tumour before it becomes clinically evident.

Therefore, the ability to detect occult metastasis is pivotal to identification of patients who would most benefit from systemic therapy and also identify patients who may be spared from unnecessary therapy."